1. Field of the Invention
The present invention relates to a microfabrication technique which may be advantageously used for manufacturing various kinds of devices, such as a magnetic bubble memory device, a semiconductor device, a surface acoustic wave device, or a Josephson device. More particularly, the present invention relates to a method for forming a pattern of a material, such as a conductive material or magnetic material, for example, on a substrate of the device, and to an apparatus adapted for being advantageously used for carrying out the above-mentioned method.
2. Description of the Related Art
In the formation of patterns in the devices as mentioned above, several important problems arise. One of these problems concerns making a fine pattern for improving the characteristics for and reducing the size or increasing the density of the device. For example, in a permalloy bubble memory device in which a bubble propagation path is defined by a permalloy pattern constituted by permalloy elements arranged with gaps therebetween, the width of the gap between the adjacent permalloy elements is one of the factors with determine the bubble propagation characteristics, and it is advantageous to make the gap width as small as possible. However, as described hereinafter, it is very difficult to form a gap of less than 1 .mu.m (submicron gap), in particular, of 0.5 .mu.m or less, using conventional techniques. Accordingly, under the existing circumstances, it is necessary to pursue improvements in the propagation characteristics and memory density of the permalloy bubble memory device by using newly-designed patterns having gaps of 1 .mu.m or more. In this connection, the T-I pattern, chevron pattern, or the like, were used at the early stages. At the present stage, however, the so-called gap-tolerant pattern, e.g., half-disk pattern and asymmetric chevron pattern, are used, whereby a bubble memory having a 2 .mu.m bubble diameter, 8 .mu.m bit period, and 1 Mbit memory density, is achieved. Furthermore, recently, the so-called wide-gap pattern has been proposed as an approach to realize a bubble memory having a 1 .mu.m bubble diameter, 4 .mu.m bit period, and 4 Mbit memory density. However, to achieve a further improvement in operating characteristic and memory density, the gaps in the permalloy pattern must be made smaller than 1 .mu.m, in particular, smaller than 0.5 .mu.m.
Further, there is a well-known ion-implanted bubble memory device in which, in order to overcome the above-mentioned gap problem, the bubble propagation path is defined by a contiguous-disk pattern formed by using an ion-implantation technique. The contiguous-disk pattern is constituted by contiguously arranged or overlapped patterns having a relatively simple shape, such as round, oval, or square. Therefore, the contiguous-disk pattern basically requires no gap for separation of the adjacent patterns and is simple in shape, thus making it possible to form a pattern having a smaller bit period than the permalloy pattern, by using a conventional technique. At present, a 4 .mu.m period pattern for the propagation of 1 .mu.m diameter bubble has been achieved. However, in achieving a contiguous-disk pattern with a period smaller than 4 .mu.m, in particular, a period of 2 .mu.m or less, the problem described below occurs.
In the contiguous-disk pattern, the shape of the cusps defined by the contiguous or overlapped disk patterns is one of the factors deciding the bubble propagation characteristics. However, a 2 .mu.m period pattern has cusps too minute to be precisely formed by using a conventional technique. It should be noted that, in the pattern forming art, it can be deemed that the cusp is a kind of gap and, accordingly, the gap forming technique described hereinafter includes the cusp forming technique.
Another problem in the forming of a pattern in the devices as mentioned above concerns a sectional configuration of the pattern. In many of these devices, a plurality of patterns are deposited one above the other. In these circumstances, to prevent inferiorities such as disconnection of the overlying pattern, a planing process must be performed by coating a resin over the underlying pattern. However, according to a conventional pattern forming method, it is impossible to control the sectional configuration of a pattern in such a manner that the sides of the pattern have a gentle slope. Accordingly, the pattern planing process cannot be easily performed.
Fine patterns, such as the permalloy pattern and contiguous-disk pattern are conventionally formed by using lithographic techniques employing light, X-rays, or an electron beam (hereinafter, generically referred to as "photolithography"). According to a conventional pattern forming method, first, a layer of material from which a pattern is formed (i.e., pattern material layer) is deposited on a surface of a substrate of a device. Next, a mask pattern used for the etching is formed of a photoresist material on the pattern material layer by photolithography. Thereafter, the pattern material layer is etched to form the pattern. According to this method, the smallest size of the formed pattern which is attainable basically depends on the limit of the mask pattern forming technique, i.e., of the photolithography. Recently, photolithography techniques have remarkably advanced, but the limit of their practical resolution is 1 .mu.m. Accordingly, it is very difficult to form a submicron gap by using the conventional pattern forming method mentioned above.
On the other hand, there are some known methods for forming a submicron gap by employing photolithography having a 1 .mu.m resolution. One of these methods uses the technique of shortening the time of the optical exposure of the pattern onto the photoresist material in the mask pattern forming process. That is, a positive photoresist material is used and the exposure time is shorter than the normal time, whereby it is possible to form a mask pattern having a submicron gap and, thus, a submicron pattern gap, under the condition of a 1 .mu.m resolution. However, the shortening of the exposure time is naturally limited, since an inferior patterning of the mask pattern is apt to occur, due to variations in the amount of light and the size of the exposed pattern.
Another known method uses an etching technique employing energized particles, for example, an ion etching technique. An ion etching process presents the phenomenon wherein a part of the material removed from the pattern material layer is redeposited onto the mask pattern and the pattern material layer and, as a result, the effect is obtained that the pattern is widened, and thus the pattern gap is correspondingly narrowed. Therefore, it is possible to form a submicron gap pattern on the basis of a 1 .mu.m gap mask pattern. However, according to the conventional pattern forming method, an effective pattern-widening effect (i.e., gap-narrowing effect) cannot be obtained, even if the ion etching technique is used, as described hereinafter.
Moreover, even if both pattern-widening effects caused by the shortening of the exposure time and the ion etching technique are combined, the smallest gap width which can be obtained by the conventional pattern forming method is 0.5 .mu.m. In addition, a very high level process control technique is required for the above, and this is not suitable to practical use.
A further known method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 54(1979)-155771. This method basically utilizes the above-mentioned pattern-widening effect caused by ion etching. That is, a pattern gap control layer is deposited on the pattern material layer, and a mask pattern is formed on the pattern gap control layer. Thereafter, the pattern gap control layer and the pattern material layer are ion-etched, so that a pattern-widening effect is obtained and, accordingly, a pattern gap which is smaller than the mask pattern gap is formed. However, this method has problems in that the process of forming the pattern gap control layer and the etching process are expensive, and processing takes a long time. And that further, the process of removing the mask pattern and the pattern gap control layer after the etching process has been completed is complicated, as described hereinafter with reference to the drawings. Therefore, this method is disadvantageous from the viewpoint of production efficiency.
Furthermore, according to the conventional and known pattern forming methods, the sectional configuration of the formed pattern depends, in principle, on the etching technique and also cannot be controlled. Therefore, a pattern formed by the conventional and known methods has a sectional configuration with the sides thereof being steeply sloped, thereby making it difficult to perform the planing process.